The present invention generally relates to a method for equalizing overall erosion of a contact arrangement in an electromagnetic switching device, and to an electromagnetic switching device corresponding to it. In particular, the xc3xa9lectromagnetic switching device may be a contactor, having a number of contacts.
Methods and the corresponding switching devices are known, for example, from DE 44 34 074 A1 and from U.S. Pat. No. 5,440,180.
Electromagnetically operated switching devices, that is to say contactors and relays, have erosion during each switching process. This is referred to in the following text as switching erosion. In this case, multipole switching devices whose main contacts switch a three-phase system are in practice subject to different contact wear. This leads to failure of the switching device, and one of the contacts becoming unserviceable. This represents a considerable restriction to the life of the switching device, because the other contacts would still frequently continue to be serviceable for some time.
This phenomenon, which is referred to as the synchronization effect, is due to the fact that the contacts which are subjected to the erosion are switched at times which are statistically not uniformly distributed with respect to the phases being switched. The synchronization effect may be caused firstly by being driven in synchronism with the power supply system or else by the switching device drive having its own response (so-called autosynchronization). Mechanical tolerances in the switching device can also result in such synchronization.
Drive systems fed with direct current and electronically controlled drive systems can be used for the switching device in order to reduce autosynchronization. Furthermore, xe2x80x9cSchaltsynchronisationseffekt bei AC-betxc3xa4tigten Schxc3xctzenxe2x80x9d [The switching synchronization effect in AC-operated contactors] (G. Griepentrog in VDE specialist report 51, 14th Contact Seminar, 09.24-26. 1997 at Karlsruhe) proposes that the drive unit for the switching device, that is to say the drive coil, be connected to a capacitance. However, this cannot prevent external synchronization processes.
In the prior art mentioned initially, it is proposed that the switching commands be delayed so as to produce a uniform distribution of the switching angles. However, when mechanical tolerances are present, this method is not able either to ensure uniform erosion of all the contacts.
An object of an embodiment of the present invention is to provide an operating method for an electromagnetic switching device, and/or to provide an electromagnetic switching device which corresponds to it. Preferably, in the method and/or switching device, uniform erosion of the contacts is always ensured.
An object may be achieved in that the respective overall erosion is determined for each contact, in that the overall erosion is supplied to a drive circuit, and/or in that the contact arrangement is operated by the drive circuit as a function of the determined (accumulated) overall erosion such that the overall erosion of the contacts are approximated to one another.
In a corresponding manner to this, an object may be achieved for the electromagnetic switching device in that the switching device has an overall erosion determining circuit via which the respective overall erosion can be determined for each contact; in that the overall erosion determining circuit is connected to, such that it can communicate with, a drive circuit; and/or in that the contact arrangement can be operated by the drive circuit as a function of the determined overall erosion, such that the overall erosion of the contacts are equalized.
The overall erosion of a contact can be the sum of the switching erosion of the contact.
Methods for determining the overall erosion are described, for example, in DE 44 27 006 A1, DE 196 03 310 A1 and DE 196 03 319 A1.
The approximation of the overall erosion of the contacts can be achieved, for example, by supplying the drive circuit with a drive command and with a reference signal which has a predetermined phase relationship with one of the three phases in a three-phase power supply system, by the contact arrangement being operated by the drive circuit with the drive command being supplied with a switching delay with respect to the reference signal, and/or by the switching delay being established by the drive circuit.
The method for approximating the overall erosion can be particularly simple when the drive circuit determines that contact which has the greatest overall erosion and the drive circuit establishes the switching delay such that the switching erosion of that contact which has the greatest overall erosion is minimized.
In this case, a control process thus takes place such that that contact which is most heavily affected by overall erosion at any given time has its load decreased by changing the switching angle until another contact has the greatest overall erosion at that time. This necessarily results in all three contacts having uniform overall erosion.
In certain types of switching devices, it is possible to carry out a series of tests in advance and hence to determine the switching erosion of the individual contacts as a function of the switching instant. This functionality can then be stored in a memory table, for example, so that a suitable switching delay for a given overall erosion can be established simply by addressing the memory table (look up).
However, it may be even better if the contact which has the greatest overall erosion couples a first phase to a three-phase load and that contact which has the medium overall erosion couples a second phase to a three-phase load. These contacts can then disconnect these respective phases from the three-phase load. The second phase may have a phase offset with respect to the first phase, and the switching delay may be increased by the phase offset.
This is because no prior knowledge of the switching device behavior, in particular of the switching erosion, may be required in this case. There is thus no need to carry out any test series. The method can be applied to all contactor types, and is self-adapting.
If the switching delay is retained for a predetermined number of drive commands after reestablishing the switching delay, the operating method operates in a particularly reliable and stable manner.
Alternatively or in addition, it is possible for the switching delay to be retained after reestablishing the switching delay until the difference between the greatest overall erosion and the least overall erosion reaches or exceeds a threshold value.
One possible way to establish the overall erosion of the contacts during normal operation is, for example, to operate the contact arrangement by a moving contact support, to detect a reference time at which the contact support assumes a reference position when the contact arrangement is opened, to record contact times at which the contacts open, and to determine the overall erosion from the differences between the contact times and the reference time.